This invention relates to a waveguide type light merging and branching device capable of electrically finely regulating wavelength characteristics of the light merging and branching device.
Light wavelength multiple transmission techniques in the communication using optical fiber are important for making a communication system more economical. A light merging and branching device is an inevitable device for the light wavelength multiple transmission. Recently a movement to investigate a waveguide type structure as a construction of the light merging and branching device for the purpose of decreasing the cost and fabricating it in a one-chip monolithic form has taken place. As an example thereof there is known a directional coupler type light branching device (N. Takato et al. : "Low-loss Directional Coupler Using High-silica Embedded Channel Waveguides" first Optoelectronics Conference (OEC '86) Technical Digest, July, 1986). As indicated in FIGS. 4A and 4B, there is disclosed a method, by which two light waveguides are arranged parallelly and light branching characteristics are obtained by utilizing the wavelength dependence of the coupling of the two light waveguides.
Since the prior art directional coupling type light branching device described above has a simple construction, it has a feature that it can be easily fabricated. However, there is a problem that the central wavelength is shifted due to variations in constructional parameters during the fabrication process. Therefore, decrease in the fabrication yield was inevitable. FIGS. 5A, 5B and 5C indicate results of the sensitivity analysis for the light branching device, which the inventors of this invention have obtained by calculations. In the figures the abscissa represents the wavelength and the ordinate the loss. FIG. 5A indicates results obtained in the case where the core width W of the waveguide is deviated by .+-.5% from a designed value of 10 .mu.m and FIG. 5B indicates those obtained in the case where the core thickness T of the waveguide is deviated by .+-.5% from a designed value of 8 .mu.m. As it can be seen from these figures, the deviations of the constructional parameters shift relatively the central wavelength. It can be understood that the wavelength shift is sensitive to the deviations in W among those described above. Such a wavelength shift gives rise to deterioration of cross talk characteristics between channels In the prior art light branching device the central wavelength shift due to variations in the constretional parameters is inevitable. Since the devices, in which the central wavelength shift has taken place, could not be used in practice, this gave rise to decrease in the fabrication yield.